Easy light polluted night sky workflow

I got four 120 seconds ISO 800 RAW frames. Better option would be to go for 60 seconds ISO 1600 and minimize tracking errors, stacking should eliminate noise anyway. Since pictures are taken under suburban sky, they are overexposed and have ugly orange background color. When we open them in Darktable default white balance preset is camera white balance and that looks like this.

Now we want to eliminate that ugly orange background and we switch from camera white balance to spot white balance.

If images were saved as JPEG, camera white balance would be applied, colors shifted to allow better compression and we won’t be able to do much processing. Before exporting images to TIFF, to export hit Ctrl+e, we will tweak exposure as on picture:

I export them as 16 bit integer per channel TIFF, if you do not know how to manage export settings it is explained in previous blog entries.
Now to do stacking I will open terminal and execute magic formula:

align_image_stack -a tif *.tiff

gmic tif0000.tif tif0001.tif -div 256 -gimp_blend 3,1,0 -mul 256 -c 0,65536 -type ushort -output one.tiff
gmic tif0002.tif tif0003.tif -div 256 -gimp_blend 3,1,0 -mul 256 -c 0,65536 -type ushort -output two.tiff
gmic one.tiff two.tiff -div 256 -gimp_blend 3,1,0 -mul 256 -c 0,65536 -type ushort -output tutorial.tiff

If you compare it with previous blog entries about G’MIC stacking you will see that new version of G’MIC is not completely backward compatible. Some people would maybe like to use DSS instead and that is also OK. Finally we open image in GIMP bump up contrast and LAB color decompose image. We duplicate A and B component, set copy mode to overlay and then merge them down. Do not flatten layers, there should be L, A and B layers. L layer can be slightly stretched or left how it is. When we LAB compose layers back into RGB image we will have nice saturated colors. Now, some people will proceed playing with curves but I will just add background gradient. I am not really using it as intended. I switch from divide to overlay and reduce opacity to 50%. That background gradient is part of astronomy plugin for GIMP by Georg Hennig and 2.8 compatible version is available from here git://gitorious.org/gimp-plugins-ambulance/gimp-plugin-astronomy.git. Building plugin is trivial. Here is result:

 
 Complete damage control was done in Darktable in three straightforward steps and that is why I call it easy. If level of black was lower and and for example exposure was reduced only -0.5EV, we could further increase contrast and get more of that Flame nebula. Though it will be more fiddling in GIMP and may be not so simple as it sounds.

Simple Moon processing workflow with Open Source tools

About 50 pictures are taken through small 4.5 inch Newtonian using T-adapter. Since back-focus on this small scope is too short, Barlow x2 is used, what rises focal ratio from f8 to f16. Out of those 50 there is one or two where turbulence is not bad. After transfer of images from camera to hard drive, Rawstudio is used to preview them and find better ones. For conversion from RAW to TIFF I am using Darktable. Here you can see what processing is done in Darktable, what modules are active:

Exported TIFF is then imported in GIMP and GMIC is activated from Filters menu. We will stay in GMIC until end with exception when we are copying layers. From Colors, Tone mapping is performed, using default parameters, and we will work further with result of that transformation.

While it looks better I am not happy with amount of detail, so from the same group with default parameters Local normalization is performed and output mode is set to new image.

Again starting from tone-mapped image, using default parameters Local contrast enhancement from Enhancement group is applied.

At this moment one can decide to use those three image to create pseudo HDR using exposure blend, but I didn’t like flat border area in output, caused by local contrast enhancement. Now I copy-paste local contrast enhanced over local contrast normalized and in GMIC, using input layers all, execute Average blending in Layers group, that is the first one with [standard]. Over result of averaging I copy-paste tone-mapped image, one we started with and do again averaging. This is final result:

Orion’s belt and sword, processing with open source tools

Capturing photos

To do processing one needs to take few photos first. Minimal hardware is DSLR with 50mm lens, remote shutter release for DSLR and any kind of cheap telescope with equatorial mount. It is not necessary to buy 50mm lens, kit lens usually 18-55mm will do, if your DSLR have live view and you can set focus. Manually setting focus on kit lens without live view is quite difficult. But it will be nice to have one 50mm F1.8 lens which is much faster and sharper than kit lens. If you have longer focal length lens it is even better, 100mm or even up to 200mm, over it tracking becomes big problem. Why remote shutter release? Camera supports up to 30 seconds exposure and after that BULB, you should press and hold shutter button what will cause lot of shaking and ruin photo. For that reason remote shutter release is used, we can have long exposure without shaking and strain. Telescope is used so that we can piggyback camera on it and achieve long exposure without trailing stars. How to attach camera to scope, there are piggyback brackets, piggyback camera mounts, you can make them on your own using hose clamp. If your telescope got RA motor drive, do polar alignment and switch it on, shorter focal length of the lens is more tolerant towards tracking errors. If there is no RA motor drive you will have to do manual tracking. Telescope with alt-azimuth mount won’t do it must be with equatorial mount. There are specialized devices like Vixen Polarie or AstroTrac which can be used instead, but they are more expensive than small 5" Newtonians with RA motor drive. Why one shouldn’t just do few hundreds shots from tripod and later stack that in Deep Sky Stacker? Because 5 seconds would be longest acceptable exposure for 50mm lens and to get to 5 minutes you need 3600 photos. Way to go is increasing exposure time.
If you are not sure where is Orion or M42 inside Orion install Stellarium http://www.stellarium.org/ it is open source and works on all major operating systems.

Processing

So I went out under my light polluted suburban sky and managed to get few decent photos between clouds. I did the same night before and now I have 13 frames 55mm F5.6 ISO 800 where exposure is from one minute to two minutes. When I decided to crop them and stack them together align_image_stack from Hugin did poor job. Was that too narrow cropping or something else I do not know, for that reason new strategy was stack them by night and later stack final results. So, imported RAWs into darktable, applied chromatic aberration and lens correction and exported them to 16 bit TIF. Should do hot pixels removal but I forgot to do it and did it later in G’MIC, like this:

gmic IMG_0451.tiff -remove_hotpixels 3,10 -c 0,65536 -type ushort -output IMG_0451hp.tiff

Now I aligned stack:

align_image_stack -a tif  *.tiff

and averaged them two by two, saving output:

gmic tif0000.tif tif0001.tif -div 256 -gimp_compose_average 1,0 -mul 256 -c 0,65536 -type ushort -output step1.tiff

Later I combined those steps, two by two, until final result. To stack those two images I need to rotate one and to crop both of them:

gmic m4291.tiff -rotate -66 -crop 1400,2500,3500,4700 -c 0,65536 -type ushort -output m4291c.tiff
gmic m4281.tiff -crop 1450,767,3550,2967 -c 0,65536 -type ushort -output m4281c.tiff

To find out how much to rotate and crop I used GIMP. Now fine alignment of those two with align_image_stack:

align_image_stack -a tif *.tiff

and final blending for16 bit output:

gmic tif0000.tif tif0001.tif -div 256 -gimp_compose_average 1,0 -mul 256 -c 0,65536 -type ushort -output m4289.tiff

and also one 8 bit for GIMP:

gmic tif0000.tif tif0001.tif -div 256 -gimp_compose_average 1,0 -output m4289.jpg

Light pollution and clouds contributed to final result.

It is too bright, too much orange in it, but M42 is visible and Flame nebula is just barely visible. If we look at histogram, this is where we are and where we want to be:

From GIMP menu we select Color->Levels. In Adjust Color Levels we set Channel to Red and move upper slide from the bottom towards middle.

We do the same for green, looking at histogram set to RGB and picture itself. After this image is still too bright and Flame nebula is invisible. To remedy that I will rise contrast, Colors->Brightness-Contrast and I set contrast on 40. Now LAB decompose color boost, described in one of previous tutorials and some more contrast stretching via Colors->Auto->White Balance and after that Edit->Fade, we want histogram stretched but not that much, again checking image and histogram to find out how much.
Here is the final result:

Conclusion I need lens and light pollution filter ;-)

How to stack images using G’MIC

After chat on Astophotography Google+ community https://plus.google.com/u/0/communities/118208937662082340807 I concluded that it may be enough interest for 16 bit image processing using G’MIC tutorial and here it is.
G'MIC stands for GREYC's Magic Image Converter and we know it as plugin for GIMP. It is less known that G'MIC framework can be used on it’s own through simple shell scripting or possible C++ or web interface. What is very important it supports 16 bits integers per channel and that is something what GIMP is lacking.
So, what we are going to do here is to take a look at G’MIC tutorial http://gmic.sourceforge.net/tutorial.shtml and load images into stack average them and save them as 16 bit TIFF.
Tutorial says if you are using G’MIC as GIMP plugin and set logging to verbose it will print what it does.

I prefer writing output to log file to suggested running GIMP from terminal (command line for Windows users). Standard path and name for log file is /tmp/gmic_log
Also from tutorial we see that 16 bit processing starts with division with 256 and ends with multiplication with 256. While that represents reduction to 8 bit it is important to note that G’MIC internally works with floating point numbers and there is no loss of precision when we convert it back to 16 bit integers. Once when we are done with processing it is important how we are going to specify TIFF output. G’MIC will pick format from extension but it by default writes 32 bit TIFF. So, to get 8 bit TIFF we need to specify output type uchar and to get 16 bit TIFF we need to specify output type ushort.
Typical operations which we are going to use are:

1.     -gimp_haar_smoothing 0.1,10,2,0,0 what is Smooth [wavelets] under Enhancements
2.     -gimp_compose_average 1,0 what is Blend [average]  under Layers
3.     -gimp_compose_screen 1,0 what is Blend [screen] under Layers

So complete workflow is we pick nicer RAWs export them as 16 bit PPMs using darktable as in http://grumpyoldprogrammer.blogspot.com/2012/11/more-about-stacking.html then we align them using align_image_stack from hugin-tools as in http://grumpyoldprogrammer.blogspot.com/2012/12/how-to-align-image-stack.html and finally we process them with G’MIC.
Averaging layers will produce at the end two layers, if we want we can save them as separate pictures, like this:

$ gmic tif0000.tif tif0001.tif tif0002.tif tif0003.tif -div 256 -gimp_compose_average 1,0 -mul 256 -c 0,65536 -type ushort -output[1] imageA.tiff -output[0] imageB.tiff

or we can stack those two layers in screen mode to boost light and stretch contrast, like this:

$ gmic tif0000.tif tif0001.tif tif0002.tif tif0003.tif -div 256 -gimp_compose_average 1,0 -gimp_compose_screen 1,0 -mul 256 -c 0,65536 -type ushort -output imageS.tiff

If we want to do resizing we can as in G’MIC tutorial use -resize2dx 1600,5 what is bi-cubic resizing to 1600 pixels width.
Instead of averaging we can try other options like -compose_median what is median or any other available filter or combination of filters, we try in GIMP, set logging to verbose and later run it in terminal without loss of data.